Intermediate Representation III

2 PAs PA2 deadline is 16.12

3 class A{ int x; int y; … } Object types x y Runtime memory layout for object of class A DispacthVectorPtr Compile time information for A class type x y 1 2 Field offsets DispacthVectorPtr 0 size ?

4 Field selection dynType(f)=A q = f.x; Move f,R1 MoveField R1. 1,R2 Move R2,q x y Compile time information for A class type Runtime memory layout for object of class A DispacthVectorPtr x y 2 Field offsets DispacthVectorPtr 0 1

5 class A{ int x; int y; … } class B extends A { int z; … } Object types and inheritance x y Runtime memory layout for object of class B z DispacthVectorPtr Compile time information for B class type x y 1 2 Field offsets DispacthVectorPtr 0 z 3 prefix of A

6 Field selection dynType(f)≤ A q = f.x; Move f,R1 MoveField R1. 1,R2 Move R2,q Runtime memory layout for object pointed by f x y ???? DispacthVectorPtr prefix of A

7 Virtual Methods class A { int x; int y; void f() {…} void g() {…} } x y Runtime memory layout for object of class A DispacthVectorPtr Compile time information for A class type f g 0 1 Method offsets x y 1 2 Field offsets DispacthVectorPtr 0

8 Method Invocation dynType(w)=A w.f() Move w,R1 VirtualCall R1.0(),Rdummy x y Runtime memory layout for object of class A DispacthVectorPtr Compile time information for A class type f g 0 1 Method offsets x y 1 2 Field offsets DispacthVectorPtr 0

Methods and Inheritance 9 class A { int x; int y; void f() {…} void g() {…} } class B extends A { int z; void f() {…} void h() {…} } f g h prefix from A Method offsets of B

10 Method Invocation dynType(w)≤ A w.f() Move w,R1 VirtualCall R1.0(),Rdummy x y ???? DispacthVectorPtr g ???? f Runtime memory layout for object pointed by w Method offsets

11 Object creation A f = new B (); |B| = |x|+|y|+|z|+|DVPtr| = = 4 (16 bytes) Label generated for class type B during LIR translation Library __allocateObject(16),R1 MoveField _B_DV, R1.0 Move R1,f

12 LIR translation example class A { int x; string s; int foo(int y) { int z=y+1; return z; } static void main(string[] args) { A p = new B(); p.foo(5); } } class B extends A { int z; int foo(int y) { s = “y=“ + Library.itos(y); Library.println(s); int[] sarr = Library.stoa(s); int l = sarr.length; Library.printi(l); return l; } }

13 LIR program (manual trans.) str1: “y=“# Literal string in program _DV_A: [_A_foo]# dispatch table for class A _DV_B: [_B_foo]# dispatch table for class B _A_foo:# int foo(int y) Move y,R1 # int z=y+1; Add 1,R1 Move R1,z Return z# return z; _B_foo:# int foo(int y) Library __itos(y),R1 # Library.itos(y); Library __stringCat(str1,R1),R2 # "y=" + Library.itos(y); Move this,R3# this.s = "y=" + Library.itos(y); MoveField R2,R3.2 MoveField R3.2,R4 Library __println(R4),Rdummy # Library.println(s); Library __stoa(R4),R5# int[] sarr = Library.stoa(s); Move R5,sarr ArrayLength sarr,R6# int l = sarr.length; Move R6,l Library __printi(l),Rdummy# Library.printi(l) Return l# return l; # main in A _ic_main:# A {static void main(string[] args)…} Library __allocateObject(16),R1 # A p = new B(); MoveField _DV_B,R1.0# Update DVPtr of new object VirtualCall R1.0(y=5),Rdummy# p.foo(5)

14 Class layout implementation class A { int x_1;... boolean x_k; void foo_1(…) {…}... int foo_n(…) {…} } x_1 = 1 … class ClassLayout { Map methodToOffset; // DVPtr = 0 Map fieldToOffset; } x_k = k foo_1 = 0 … foo_n = n-1 fieldToOffset methodToOffset _DV_A: [foo_1,…,foo_n] lir file MoveField R1.3,R9 VirtualCall R1.7(),R3

15 Representing arrays/strings str1: “Hello!” 3123 R1:[1,2,3] 1 word = 4 bytes

16 LIR optimizations Aim to reduce number of LIR registers and number of instructions Avoid storing variables and constants in registers Use accumulator registers Reuse “dead” registers Weighted register allocation

17 Avoid storing constants and variables in registers Don’t allocate target register for each instruction TR[5] = Move 5,Rj TR[x] = Move x,Rk For a constant TR[5] = 5 For a variable TR[x] = x TR[x+5] = Move 5,R1 Add x,R1 Translation for a “simple” sub tree

18 Accumulator registers Use same register for sub-expression and result TR[e1 OP e2] R1 := TR[e1] R2 := TR[e2] R3 := R1 OP R2 R1 := TR[e1] R2 := TR[e2] R1 := R1 OP R2

19 Accumulator registers TR[e1 OP e2] c+(b*a) R1 := TR[e1] R2 := TR[e2] R3 := R1 OP R2 R1 := TR[e1] R2 := TR[e2] R1 := R1 OP R2 Move b,R1 Mul a,R1 Add c,R1 Move a,R1 Move b,R2 Mul R1,R2 Move R2,R3 Move c,R4 Add R3,R4 Move R4,R5

20 Accumulator registers cont. For instruction with N registers dedicate one register for accumulation Accumulating instructions, use: MoveArray R1[R2],R1 MoveField R1.7,R1 StaticCall _foo(R1,…),R1 …

21 Reuse registers Registers have very-limited lifetime TR[e1 OP e2] = R1:=TR[e1] R2:=TR[e2] R1:=R1 OP R2 Registers from TR[e1] can be reused in TR[e2]

22 Weighted register allocation Sethi & Ullman algorithm Two expression e1, e2 and an operation OP e1,e2 without side-effects TR[e1 OP e2] = TR[e2 OP e1] Weighted register allocation translate heavier sub-tree first

23 Example R0 := TR[a+(b+(c*d))] b cd * + + a R0 R1 R2 Translation uses all optimizations shown until now uses 3 registers R2 R1 left child first b cd * + + a R0 Managed to save two registers R0 right child first R0

24 Weighted register allocation Can save registers by re-ordering subtree computations Label each node with its weight Weight = number of registers needed Leaf weight known Internal node weight w(left) > w(right) then w = left w(right) > w(left) then w = right w(right) = w(left) then w = left + 1 Choose heavier child as first to be translated Have to check that no side-effects exist

25 Weighted reg. alloc. example b 5c * array access + a baseindex W=1W=0W=1 W=2 Phase 1: - check absence of side-effects in expression tree - assign weight to each AST node R0 := TR[ a+b[5*c] ]

26 Weighted reg. alloc. example R0 := TR[ a+b[5*c] ] b 5c * array access + a R0 baseindex W=1 W=0W=1 W=2 R1 Phase 2: use weights to decide on order of translation Heavier sub-tree Move c,R0 Mul 5,R0 Move b,R1 MoveArray R1[R0],R0 Add a,R0

27 microLIR simulator Written by Roman Manevich Java application Accepts file.lir Executes program Use it to test your translation Checks correct syntax Performs lightweight semantic checks Runtime semantic checks Debug modes (-verbose:1|2) Prints program statistics (#registers, #labels, etc.) Comes with sample inputs Read manual